1. Field of the Invention
The present invention relates to an intake air control apparatus and an intake air control method for an internal-combustion engine.
2. Description of the Related Art
An example of an existing control apparatus for an internal-combustion engine is described in Japanese Unexamined Patent Application Publication No. 2004-225612. This internal-combustion engine includes a port fuel injection valve for injecting fuel to an intake port. In addition, the control apparatus performs control to increase fuel supplied to the internal-combustion engine when the opening of a throttle valve is larger than predetermined reference opening. At that time, the increase in fuel is set to a larger value as the rotation speed of the engine increases and as the amount of basic fuel consumption increases. The increased fuel cools the internal-combustion engine and a catalyst. In this way, overheating of the internal-combustion engine and catalyst can be prevented.
However, if the amount of the supplied fuel is increased in the above-described manner, the amount of the supplied fuel is further increased from that in a high rotation and high load operation in which fuel is highly consumed. At that time, an existing control apparatus only sets an increase in supplied fuel in accordance with the rotation speed of the engine and the amount of basic fuel consumption. Accordingly, the fuel injection quantity including the increased amount may exceed a maximum fuel quantity suppliable by the fuel injection valve (hereinafter, referred to as a “maximum fueling rate”). Thus, the required fuel quantity may not be supplied. In such a case, it is difficult to prevent overheating of a catalyst. In addition, the air-fuel ratio of the mixture of air and fuel may be disadvantageously shifted from the desired air-fuel ratio.
For example, in the case of a port fuel injection valve as in existing internal-combustion engines, fuel injection can be made at any point in time during one combustion cycle. Accordingly, the fuel can be supplied within a long allowable period of time. However, in the case of a direct injection valve that directly injects fuel into a cylinder of an internal-combustion engine, the effective injection period is limited to, for example, a compression stroke period. Thus, since the effective injection period is significantly short, a required fuel quantity may not be supplied. Alternatively, when the quality of the fuel used is low and, therefore, engine knock easily occurs, the ignition timing needs to be retarded in order to prevent engine knock. Thus, the catalyst easily overheats. Accordingly, in order to prevent overheating, an increase in controlled fuel quantity needs to be large. As a result, the same problem easily occurs.